In order to reduce an operation of a vehicle by a driver, a vehicle drive control device for executing an automatic drive control, such as a cruise control for executing a constant vehicle speed control by which a vehicle speed is controlled to reach a target vehicle speeds an adaptive cruise control (ACC) by which the vehicle is controlled so as to follow a vehicle traveling in front of the subject vehicle and the like, is adapted to the vehicles. In the vehicle drive control device, an engine functioning as a driving torque generating device, which generates a driving torque, and a brake device generating a braking torque are cooperatively controlled so that the vehicle is driven at a preset target vehicle speed. Further, according to the vehicle drive control device, an automatic drive control electronic control unit (hereinafter referred to ECU) calculates a target driving torque so that the vehicle speed reaches the target vehicle speed. The calculated target driving torque is outputted to an engine ECU, then the engine ECU controls the engine based on the target driving torque. Moreover, according to the vehicle drive control device, the automatic drive control ECU calculates a target braking torque so that the vehicle speed reaches the target vehicle speed. The calculated braking torque is outputted to a brake ECU, then the brake ECU controls the braking device based on the target braking torque.
A conventional automatic drive control for controlling the vehicle to be driven at a low target speed, for example, equal to or less than 10 km/h is known. For instance, according to a known vehicle drive control device disclosed in JP2004-90679A (which is hereinbelow referred to as a reference 1), the automatic drive control is executed so as to control the vehicle to be driven at a target speed as low as a creep speed. According to the conventional vehicle drive control device disclosed in reference 1, even though the driver operates a brake, the automatic drive control is not canceled and remains to be executed. According to the conventional vehicle drive control device, for example, when the driver operates the brake, the automatic drive control is executed so the engine does not generate the driving torque.
On the other hand, when the driver cancels the braking operation, the vehicle drive control device executes the automatic drive control so that the engine generates the driving torque to drive the vehicle at the target speed. In other words, when the driver cancels the braking operation, the vehicle drive control device increases the driving torque which is generated by the engine so as to control the vehicle speed to reach the target speed. The brake device generates friction as the braking torque by contacting pads with rotors by an application of, for example a hydraulic pressure of the wheel cylinders. Therefore, when the driver cancels the braking operation in a condition in which the braking torque is generated by the hydraulic pressure of the wheel cylinders in response to the braking operation of the driver, the hydraulic pressure of the wheel cylinders is decreased, so that the braking torque is decreased. However, immediately after the driver cancels the braking operation, the hydraulic pressure of the wheel cylinders may not be decreased sufficiently and may remain at wheel cylinders. In a case in which the hydraulic pressure of the wheel cylinders remains at wheel cylinders immediately after the driver cancels the braking operation, the braking torque is generated by the remaining hydraulic pressure. Therefore, when the automatic drive control is executed in such a condition so that the vehicle speed reaches the target vehicle speed, the driving torque generated by the engine is higher than a condition in which the hydraulic pressure of wheel cylinders does not remain at the wheel cylinders. Accordingly, when the hydraulic pressure of the wheel cylinders does not remain (i.e. when the hydraulic pressure of the wheel cylinders is removed) after the driver cancels the braking operation, the braking torque reaches zero, and the increasing driving torque may result in that the vehicle speed exceeds the target vehicle speed and the vehicle suddenly accelerates.
A need thus exists for a vehicle drive control device which is not susceptible to the drawback mentioned above.